Sahih Muslim Book 003, Hadith Number 0671.
Chapter : Utmost care for keeping private parts of body concealed.
Jabir b. 'Abdullah reported: The Messenger of Allah (may peace be upon him) was carrying along with them (his people) stones for the Ka'ba and there was a waist wrapper around him. His uncle, "Abbas, said to him: O son of my brother! if you take off the lower garment and place it on the shoulders underneath the stones, it would be better. He (the Holy Prophet) took it off and placed it on his shoulder and fell down unconscious. He (the narrator) said: Never was he seen naked after that day.